Resistor



G. B. CROUSE Oct. 24, 1933.

RESISTOR Original Filed July 24, 1930 INVENTOR George B. Crouse Wdfi: i'f f ATTC Y Patented Oct. 24, 1933 UNITED STATES PATENT OFFICE RESISTOR ration of New Jersey Original application July 24, 1930, Serial No.

470,380, Patent 1,880,239, October 4,

Divided and this application November 25, 1931.

Serial No. 577,304

2 Claims. (01. 201-64) This invention relates to resistors and. more particularly to those of the tubular, vitreous enamel coated type.

In general a vitreous enamel resistor comprises a tube of porcelain or other refractory material, around the ends of whch are clamped suitable contact bands of copper or other conducting material. The resistance wire, which is usually an alloy having high electrical resistance, is then wound on the tube between the contact bands and riveted or otherwise attached thereto at its ends. The wound tube is then dipped or sprayed with the vitreous enamel slip and fired at such a temperature that the enamel becomes molten and flows over the entire surface of wire and tube. One or more coats of enamel, as required, are applied and fired over the original coat.

It is obvious that ina resistor of this type,

since the entire wire is coated with enamel, it is impossible to make any adjustments of the resistance value or to take off acontact at any point after the resistor is completed. This feature is disadvantageous from several angles. In the first place, even though the original resistor is wound accurately to a specified resistance value, oxidation of the wire will take place in the firing process, as a result of which the wire is reduced in cross-section and its resistance increased by an amount which is not accurately predictable; Secondly, in many applications of a resistor it is desirable to be able to change the resistance in service or to use the device as an adjustable potentiometer;

Many attempts have been made in the past to produce a resistor of the type to which this invention is directed, none of which have been conspicuously successful. Since the finished enamel coating is very hard and brittle, it is impossible to satisfactorily grind it off alongthe section in which it is desired to exposethe wire. Due to variations in the amount by which the turns of wire project from the tube, the grinding operation will take off more or less of the material of those turns which extend beyond the others with consequent reduction of their cross sectional area.

and variations in resistance throughout the wire.

In view of these disadvantages most of the previous attacks on the problem were directed to exposing a track before firing. .The common method was as follows: Before the wound resistor was dipped or sprayed in the enamel slip, a shield, generally of adhesive tape, was laid along the resistor, the enamel thus being preventedfrom reaching the area which it was proposed to expose. The disadvantages of this method are twofold: In the first place, the fluid enamel in the furnace, during the firing process, has a very high surface tension and it therefore pulls back away from that area which was shielded and leaves a larger and irregularly edged area exposed. This produces a unit of very poor appearance and has other disadvantages, as will be seen immediately. Secondly, the wires are entirely exposed and unsupported in the slot. As a result of this, any form of sliding contact that may be used with the wires tends to displace them, bend them, and ultimately to break them.

The general object of the invention is to pro vide a vitreous enamel resistor having an area in which the wires are exposed, so that contact may be made with them at any desired point in this area. Except where the wires are exposed, they are completely embedded in the enamel. At the exposed area the enamel after the firing operation, keeps the turns properly spaced and holds them firmly against displacement when a slidable contact device is used with the resistor in a rheostat.

The particular nature of the invention, as well as other objects and advantages thereof, will appear most clearly from a description of certain preferred embodiments thereof as shown in the accompanying drawing in which Fig. 1 is a view of the entire resistor showing the exposed track;

Fig. 2 is a transversesectional view of the resistor taken along the line 22 of Fig. 1 on an enlarged scale, and 4 Fig. 3 is a similar view in longitudinal section along the line 3--3 of Fig. 2.

I The article may be produced in accordance with the method forming the subject matter of my copending application Serial No. 470,380, for Resis-v tors, filed July 24, 1930, which issued as Patent 1,880,239, October 4, 1932, of which application this application is a division. In this a form 1, such as a tube of refractory material, is wound with suitable wire 2, the ends of which are attached to contact bands 3, the ends of which extend outwardly to serve as terminals 4 and are suitably fastened together. The resistance unit thus produced is completely coated with suitable enamel slip 5 in the usual way and then allowed to partially dry in the air.

The enamel slip is brushed off to expose the wire along a track 6. The unit is then removed and. is ready to be fired. The required number of coats having been applied in accordance with the procedure described above, it will be found that there is a very thin skin of oxide and enamel on removed by rubbing the exposed portion with emery cloth or other suitable abrasive. Very little effort is required to give a bright surface to the exposed wire in the track 6.

The appearance of the resulting resistor is best illustrated in Figs. 2 and 3 wherein the exposed outer surfaces of the wire 2 between the edges of the enamel 5 at the sides of the track 6 are shown as projecting slightly above the enamel which during the brushing operation has re:- mained between the turns of the wire. During the firing of the resistor the outer surface of the enamel between the turns of wire, due to the effect of surface tension, assumes a curved contour as shown most clearly at '7 in Fig. 3. The enamel between the turns holds them firmly against displacement when a slidable contact device is used with the wire as in a rheostat. Since it is unnecessary to remove any of the enamel after it has been fired, there is no danger of any reduction of the cross-section of the wire at any points.

In the manufacture of resistors of the type described in this specification, it is found very desirable to lise a refractory tube having as high a coeflicient of expansion as obtainable. In addition to which the ,wire should be wound on the tube under a high tension. If these precautions are not observed, it will be'found that the turns of wire will expand away from the tube in the firing process and the surface tension of the enamel may pull the turns apart in some places 1,931,981 the exposed portions of the wire. This may be and bunch them together in others, making an uneven spacing of turns in the finished unit.

While a preferred embodiment of the invention has been disclosed, it will be understood that the resistors may be of various forms having exposed areas of various shapes and that various changes in the details of construction of the devices shown herein as well as modifications of the article .described herein may be made without departing from the principles of invention as defined in the appended claims.

I claim:

1. A resistor consisting of a refractory form having a wire wound thereon, with a portion of the wire completely embedded in vitreous enamel and the remainder of the wire being partially embedded in the enamel with the outer portion of its turns exposed, the outer surface of the enamel between. said exposed turns having a curved contour.

2. A resistor consisting of a refractory form having a wirewound thereon with a portion of the wire completely embedded in vitreous enamel and the remainder of the wire being partially embedded in the enamel along an area extending longitudinally of the resistor with the outer pertions of its turns exposed to form a track, the

outer surface of the enamel between the track of exposed turns having an inwardly curved contour.

GEORGE B. CROU SE. 

